The present invention relates generally to inverter systems and more particularly to an inverter system for supplying electrical power to a motor, which system includes novel circuitry for the detection and correction of commutation faults, often and more popularly called "shoot-throughs".
The variable power conversion unit currently most popular, whether direct current (d.c.) to alternating current (a.c.) or a.c. to d.c., employs a bridge arrangement of controlled rectifiers. Today's most commonly used controlled rectifier is of the semiconductor type, generically known as the thyristor, the most common form of which is the silicon controlled rectifier (SCR). In the remaining portion of this specification, the word "thyristor" will be used in that generic sense and it is understood that this term is used to designate controlled rectifiers generally. The three phase version of the power conversion unit normally includes six thyristors in the customary bridge arrangement such that there are two series connected thyristors in each of three legs. These thyristors are normally rendered conductive in a prescribed sequence to control power from the source to the load. There are, however, occasions where one or more of the thyristors will fail to commutate or turn off at the proper time, thus resulting in an improper conductive situation. Viewed from the d.c. side of the bridge, when a thyristor fails to commutate there exists a direct short circuit across the d.c. buses because both thyristors of a leg are in the conductive condition. This is known as a commutation fault or more commonly, a shoot-through.
There are many causes of shoot-throughs but, regardless of origin, the ultimate cause is the failure of the thyristor current to reduce to a value where the thyristor will cease to conduct. The effective of the shoot-through on system performance varies with the type of inverter used. In the case of a voltage source inverter, a shoot-through generally requires the inverter to be shut down. In a current source inverter, so long as the shoot-through is of short duration, there is usually no adverse effect on either the thyristor or the overall control of the power supplied to the load. The majority if not most systems, therefore, include some form of shoot-through protection to detect and take corrective action when a shoot-through is imminent or has already occurred. This protective action can and does take on a variety of forms, the ultimate purpose of all being to reduce the thyristor current to a point where the thyristor will cease to conduct. The form of the particular system is often governed to a large degree by the nature of the conversion bridge and its control as well as, or in addition to, the nature of the load itself. Many such protection schemes are very complex and hence expensive. This is particularly true in very closely or precisely controlled systems which employ anticipatory schemes in an attempt to detect an incipient shoot-through and take preventive action with respect thereto before the shoot-through actually occurs. In other applications, however, the expense of such a scheme is not warranted since the existence of a shoot-through, so long as it is not allowed to continue, does not seriously affect the overall system performance. As an example, in extremely large motor drives where the inertia of the system inherently makes the response time of the system relatively slow, transient shoot-throughs which are corrected within a portion of a cycle do not seriously or adversely affect overall system performance.